1. Field of the Invention
The present invention generally relates to magnetic microspheres for use in fluorescence-based applications. Certain embodiments relate to a microsphere that includes a magnetic material coupled to a surface of a core microsphere and a polymer layer surrounding the magnetic material and the core microsphere.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Magnetic microspheres are currently used in a wide variety of applications, including: hyperthermic treatment of tumors; directed delivery of therapeutic substances to target locations in living systems; cell, polynucleotide, and protein isolation; and clinical analysis of biomolecules. Microspheres suitable for such purposes are available from a number of commercial sources in a number of different configurations. These microspheres often include a magnetically susceptible substance and a spherical matrix material such as an organic polymer or silica. The microspheres may have several configurations such as a magnetic core surrounded by a matrix; small magnetic particles dispersed throughout a matrix; and a magnetic coating on the outside of a spherical matrix. Each of these microsphere configurations has advantages and disadvantages, and selection of the appropriate configuration is dependent on the intended use of the microspheres.
For many purposes, appropriate microspheres display paramagnetism or superparamagnetism, rather than ferromagnetism. Such microspheres have negligible magnetism in the absence of a magnetic field, but application of a magnetic field induces alignment of the magnetic domains in the microspheres, resulting in attraction of the microspheres to the field source. When the field is removed, the magnetic domains return to a random orientation so there is no interparticle magnetic attraction or repulsion. In the case of superparamagnetism, this return to random orientation of the domains is nearly instantaneous, while paramagnetic materials will retain domain alignment for some period of time after removal of the magnetic field. This retention of domain alignment may lead to microsphere aggregation in the absence of an external magnetic field, which is often undesirable. Ferromagnetic materials have permanently aligned domains, so microspheres including such magnetic materials will readily aggregate.
The matrix material associated with the magnetic material also varies depending on the intended use of the microspheres, with silica and polymer latex being the most commonly used matrix materials. Both materials can be used to create substantially uniform magnetic microspheres in a wide range of diameters. Magnetic silica microspheres are often more stable over a wider range of temperatures than microspheres made from organic polymers such as polystyrene, and both materials may decompose in certain environments such as acidic or aromatic solvents. Additionally, silica microspheres are often more dense than latex microspheres, which can be an important consideration in the choice of magnetic microsphere matrices.
A significant and growing use of magnetic microspheres is in the field of biological assays. Assays for proteins and oligonucleotides can be performed on the surface of the microspheres, which can then be magnetically separated from the reaction mixture before the characteristics of the microspheres are measured. Isolation of the assay microspheres prior to measurement decreases interference of non-target molecules with the measurements thereby producing more accurate results.
Concurrent with the increasing interest in magnetic microspheres for biological assays is the development of assays conducted on fluorescent microspheres. The use of fluorescent labels or fluorescent material coupled to a surface of the microspheres or incorporated into the microspheres allows preparation of numerous sets of microspheres that are distinguishable based on different dye emission spectra and/or signal intensity. In a biological assay, the fluorescence and light scattering of these microspheres can be measured by a flow cytometer or an imaging system, and the measurement results can be used to determine the size and fluorescence of the microspheres as well as the fluorescence associated with the assay system being studied (e.g., a fluorescently labeled antibody in a “capture sandwich” assay), as described in U.S. Pat. No. 5,948,627 to Lee et al., which is incorporated by reference as if fully set forth herein. By varying the concentrations of multiple dyes incorporated in the microspheres, hundreds, or even thousands, of distinguishable microsphere sets can be produced. In an assay, each microsphere set can be associated with a different target thereby allowing numerous tests to be conducted for a single sample in a single container as described in U.S. Pat. No. 5,981,180 to Chandler et al., which is incorporated by reference as if fully set forth herein.
Fluorescently distinguishable microspheres may be improved by rendering these microspheres magnetically responsive. Examples of methods for forming fluorescent magnetic microspheres are described in U.S. Pat. No. 5,283,079 to Wang et al., which is incorporated by reference as if fully set forth herein. The methods described by Wang et al. include coating a fluorescent core microsphere with magnetite and additional polymer or mixing a core microsphere with magnetite, dye, and polymerizable monomers and initiating polymerization to produce a coated microsphere. These methods are relatively simple approaches to the synthesis of fluorescent magnetic microspheres, but are not suitable for creating the large numbers of precisely dyed microspheres used in relatively large multiplex assays such as those as described in U.S. Pat. No. 5,981,180 to Chandler et al.
This limitation of the methods of Wang et al. is due to the fact that most fluorescent dye molecules are extremely sensitive to attack by radical species generated during radical initiation polymerizations. If these radicals inactivate even a relatively small number of dye molecules, precise quantities of dye in the microspheres cannot be achieved. Furthermore, if the methods of Wang et al. are used to synthesize non-fluorescent magnetic microspheres, and dyeing of the microspheres is attempted using the solvent swelling method described in U.S. Pat. No. 6,514,295 to Chandler et al., which is incorporated by reference as if fully set forth herein, a relatively large amount of the magnetic material will be released from the microspheres during dyeing since the magnetic material is not chemically bound to the microspheres. In particular, physical entrapment of the magnetic material in the microspheres will be disrupted by the swelling process, and the magnetic material will be released into solution.
Fluorescent magnetic microspheres are also described in U.S. Pat. No. 6,268,222 to Chandler et al., which is incorporated by reference as if fully set forth herein. In this method, nanospheres are coupled to a polymeric core microsphere, and the fluorescent and magnetic materials are associated with either the core microsphere or the nanospheres. This method produces microspheres with desirable characteristics, but the nanosphere-microsphere bond may be susceptible to cleavage under severe reaction conditions. A coating surrounding the microsphere and nanospheres bound thereto may be used to improve this association but, again, the use of radical initiators to form this coating can compromise the fluorescent emission profile of the microsphere.
A more desirable configuration for a fluorescent magnetic microsphere is a magnetically responsive microsphere that can be dyed using established techniques, such as those described in U.S. Pat. No. 6,514,295 to Chandler et al. In general, this method uses solvents that swell the microsphere thereby allowing migration of the fluorescent material into the microsphere. These dyeing solvents include one or more organic solvents. Therefore, the microspheres must be able to tolerate organic solvents without losing their compositional integrity. Additionally, free magnetite interferes with a number of biological reactions. Therefore, microspheres that are susceptible to loss of even a relatively small amount of magnetite are unacceptable. As such, magnetic microspheres should be constructed such that the magnetic material is tightly bound to the microspheres thereby preventing loss of the magnetic material during the swelling process.
Magnetic microspheres are described in U.S. Pat. Nos. 5,091,206 to Wang et al., 5,648,124 to Sutor, and 6,013,531 to Wang et al., which are incorporated by reference as if fully set forth herein. However, the microspheres produced by the methods disclosed in these patents to Wang et al. may lose magnetic material when disposed in organic solvents since the magnetic material is not chemically immobilized. In each of these patents, substantially small particles of magnetic material are coated on a polymeric core microsphere, with a polymeric shell as an outer coating. The magnetic particles are synthesized and processed to minimize particle size. Some magnetic materials, such as Fe3O4, progress from ferromagnetic to paramagnetic to superparamagnetic as particle size decreases. Therefore, magnetite particles having the smallest size possible are used to form the microspheres such that the product microsphere shows little magnetic retentivity.
However, if the product microsphere is also configured to emit a fluorescent signal from dye substances in the core microsphere, the thickness of the layer of magnetic particles on the surface of the microsphere core becomes an important consideration. For example, since most magnetic substances are optically opaque, a relatively thick coating of magnetic particles on the surface of the microsphere core will cause excessive light scatter or blocking of photon transmission. Using the methods of these patents, in which the magnetic component is designed to have the minimum particle size, magnetic content would have to be limited to allow light transmission through the magnetic component. In fact, to provide a magnetic microsphere having a diameter of 7 μm and 5% magnetic content, the entire surface of the core microsphere would have to be coated with a layer of magnetite having a thickness of 15 nm. This thickness results in a significantly lower fluorescent signal. Even if some of the magnetic particles are larger, as described by Sutor, the presence of a relatively large number of smaller magnetic particles will strongly impact the emission profile of the microsphere. One object of the invention of Sutor is to provide magnetically responsive microparticles having more electro magnetic units (EMUs) per gram of material than known microparticles.
The previously used methods also use surfactants and stabilizers in the preparation of the outer coatings. For many purposes, the presence of these molecules on the surface of the microsphere is acceptable. However, when used in bioassays, the surfactants can result in unwanted interference and changes in the binding efficiency of biomolecules to the microsphere surface. Washing procedures may reduce the quantity of surfactants and stabilizers associated with the microsphere surface, but completely removing the surfactants and stabilizers is extremely difficult.
Therefore, it would be a significant improvement over existing technologies to provide methods for forming a microsphere containing greater than about 2% by weight of a magnetically responsive material, without significantly hindering light transmission into and out of the microsphere. It would be a further improvement if this magnetically responsive material is strongly associated with the microsphere thereby reducing loss of the magnetically responsive material during dyeing and is surrounded by a polymer to substantially prevent the magnetically responsive material from interacting with biomolecules of interest. Furthermore, previously used methods can be improved if the outermost polymer layer is formed in the absence of surfactants and stabilizers.